The present invention relates to a digital signal transmission system, for use in a communication system in which a digital signal is transmitted in a time division multiplexing mode over a communication cable, wherein sound transmission can be effected with multifunctions.
The basic sound transmission mode is a one-to-one communication mode using analog signals, in which subscriber lines are coupled by a line switching network, as typically represented by a common telephone system. This applies as well to a small-scale system, such as a private or premise communication network.
Meanwhile, it has been demanded that a telephone be made multifunctional in order to improve productivity in an office. The development of facsimile transmission through telephone lines is an example of such multifunctionality. In attaining such multifunctionality of the telephone, however, the basic transmission mode for conventional one-to-one communication must still be effected. Accordingly, for example, where one telephone line has been commonly used for both telephone and facsimile transmission it has been ordinary that the telephone line has been commonly, but exclusively, used. That is, ordinarily, the telephone could not be used during facsimile transmission, and vice versa. Further, where such a line has been connected between two or more subscribers, it has been ordinarily impossible for one of these subscribers to communicate directly with another. In this case, it has been necessary, for example, for one of the subscribers to establish a further communication channel using another telephone set and another line.
In the conventional sound transmission communication system whose basic transmission mode involves one-to-one communication, it has been difficult to sufficiently provide multifunctional sound transmission.